Add support for assistant conversational mode in audioserver

This is a new mode that will inform the audio framework that an active
assistant conversation is currently happening. This can have influence
on the volume management or routing.

Test: m
Bug: 416329698
Flag: android.media.audio.assistant_volume_control
Change-Id: I68f38de16111d9c2dc02e06fe7300a59df21d6b2